WordPress is a fantastic system in many ways, but one place that it’s really lacking is the ability to extend existing plug-ins. There is no built-in system of dependency when it comes to WordPress plug-ins, unfortunately.

Therefore, if you’re thinking of adding on to an existing WordPress plug-in, you basically have two options:

You can modify the plug-in itself
Using this method isn’t all that ideal, because any changes you make will obviously be overwritten whenever the plug-in is updated.

You can write a plug-in that attempts to depend on the other plug-in
With this method, if something changes in the other plug-in that causes your dependency check to fail, you could end up breaking the WordPress installation (which results in a blank white screen on most installations)

I would love to see WordPress implement some sort of dependency check or prioritizing method for plug-ins similar to the way they’ve implemented the method of using javascript and stylesheets. Sadly, though, there seems to be opposition from the development team because of too many unforeseen variables in the process.